Stent fabrication via tubular casting processes

ABSTRACT

Tubular casting processes, such as dip-coating, may be used to form substrates from polymeric solutions which may be used to fabricate implantable devices such as stents. The polymeric substrates may have multiple layers which retain the inherent properties of their starting materials and which are sufficiently ductile to prevent brittle fracture. Parameters such as the number of times the mandrel is immersed, the duration of time of each immersion within the solution, as well as the delay time between each immersion or the drying or curing time between dips and withdrawal rates of the mandrel from the solution may each be controlled to result in the desired mechanical characteristics. Additional post-processing may also be utilized to further increase strength of the substrate or to alter its shape.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/476,853 filed May 21, 2012, which is a divisional of U.S. patentapplication Ser. No. 12/143,659 filed Jun. 20, 2008 (now U.S. Pat. No.8,206,635), the content of each of which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to manufacturing processes forforming or creating devices which are implantable within a patient, suchas medical devices. More particularly, the present invention relates tomethods and processes for forming or creating tubular substrates whichmay be further processed to create medical devices having variousgeometries suitable for implantation within a patient.

BACKGROUND OF THE INVENTION

In recent years there has been growing interest in the use of artificialmaterials, particularly materials formed from polymers, for use inimplantable devices that come into contact with bodily tissues or fluidsparticularly blood. Some examples of such devices are artificial heartvalves, stents, and vascular prosthesis. Some medical devices such asimplantable stents which are fabricated from a metal have beenproblematic in fracturing or failing after implantation. Moreover,certain other implantable devices made from polymers have exhibitedproblems such as increased wall thickness to prevent or inhibit fractureor failure. However, stents having reduced wall thickness are desirableparticularly for treating arterial diseases.

Because many polymeric implants such as stents are fabricated throughprocesses such as extrusion or injection molding, such methods typicallybegirt the process by starting with an inherently weak material. In theexample of a polymeric stent, the resulting stent may have imprecisegeometric tolerances as well as reduced wall thicknesses which may makethese stents susceptible to brittle fracture.

A stent which is susceptible to brittle fracture is generallyundesirable because of its limited ability to collapse for intravasculardelivery as well as its limited ability to expand for placement orpositioning within a vessel. Moreover, such polymeric stents alsoexhibit a reduced level of strength. Brittle fracture is particularlyproblematic in stents as placement of a stent onto a delivery balloon orwithin a delivery sheath imparts a substantial amount of compressiveforce in the material comprising the stent. A stent made of a brittlematerial may crack or have a very limited ability to collapse or expandwithout failure. Thus, a certain degree of malleability is desirable fora stent to expand, deform, and maintain its position securely within thevessel.

Accordingly, it is desirable to produce a polymeric substrate having oneor more layers which retains its mechanical strength and is sufficientlyductile so as to prevent or inhibit brittle fracture, particularly whenutilized as a biocompatible and/or bioabsorbable polymeric stent forimplantation within a patient body.

SUMMARY OF THE INVENTION

A number of casting processes described herein may be utilized todevelop substrates, e.g., cylindrically shaped substrates, having arelatively high level of geometric precision and mechanical strength.These polymeric substrates can then be machined using any number ofprocesses (e.g., high-speed laser sources, mechanical machining, etc.)to create devices such as stents having a variety of geometries forimplantation within a patient, such as the peripheral or coronaryvasculature, etc.

An example of such a casting process is to utilize a dip-coatingprocess. The utilization of dip-coating to create a polymeric substratehaving such desirable characteristics results in substrates which areable to retain the inherent properties of the starting materials. Thisin turn results in substrates having a relatively high radial strengthwhich is retained through any additional manufacturing processes forimplantation. Additionally, dip-coating the polymeric substrate alsoallows for the creation of substrates having multiple layers.

The molecular weight of a polymer is typically one of the factors indetermining the mechanical behavior of the polymer. With an increase inthe molecular weight of a polymer, there is generally a transition frombrittle to ductile failure. A mandrel may be utilized to cast ordip-coat the polymeric substrate.

In dip-coating the polymeric substrate, one or more high molecularweight biocompatible and/or bioabsorbable polymers may be selected forforming upon the mandrel. The one or more polymers may be dissolved in acompatible solvent in one or more corresponding containers such that theappropriate solution may be placed under the mandrel. As the substratemay be formed to have one or more layers overlaid upon one another, thesubstrate may be formed to have a first layer of a first polymer, asecond layer of a second polymer, and so on depending upon the desiredstructure and properties of the substrate. Thus, the various solutionsand containers may be replaced beneath the mandrel between dip-coatingoperations in accordance with the desired layers to be formed upon thesubstrate such that the mandrel may be dipped sequentially into theappropriate polymeric solution.

Parameters such as the number of times the mandrel is immersed, thesequence and direction of dipping, the duration of time of eachimmersion within the solution, as well as the delay time between eachimmersion or the drying or curing time between dips and dipping and/orwithdrawal rates of the mandrel to and/or from the solution may each becontrolled to result in the desired mechanical characteristics.Formation via the dip-coating process may result in a polymericsubstrate having half the wall thickness while retaining an increasedlevel of strength in the substrate as compared to an extruded polymericstructure.

The immersion times as well as drying times may be uniform between eachimmersion or they may be varied as determined by the desired propertiesof the resulting substrate. Moreover, the substrate may be placed in anoven or dried at ambient temperature between each immersion or after thefinal immersion to attain a predetermined level of crystals, e.g., 60%,and a level of amorphous polymeric structure, e.g., 40%. Each of thelayers overlaid upon one another during the dip-coating process aretightly adhered to one another and the wall thicknesses and mechanicalproperties of each polymer are retained in their respective layer withno limitation on the molecular weight and/or crystalline structure ofthe polymers utilized.

Dip-coating can be used to impart an orientation between layers (e.g.,linear orientation by dipping; radial orientation by spinning themandrel; etc.) to further enhance the mechanical properties of theformed substrate. As radial strength is a desirable attribute of stentdesign, post-processing of the formed substrate may be accomplished toimpart such attributes. Typically, polymeric stents suffer from havingrelatively thick walls to compensate for the lack of radial strength,and this in turn reduces flexibility, impedes navigation, and reducesarterial luminal area immediately post implantation. Post-processing mayalso help to prevent material creep and recoil (creep is atime-dependent permanent deformation that occurs to a specimen understress, typically under elevated temperatures) which are problemstypically associated with polymeric stents.

For post-processing, a predetermined amount of force may be applied tothe substrate where such a force may be generated by a number ofdifferent methods. One method is by utilizing an expandable pressurevessel placed within the substrate. Another method is by utilizing abraid structure, such as a braid made from a super-elastic or shapememory alloy like NiTi alloy, to increase in size and to apply thedesirable degree of force against the interior surface of the substrate.

Yet another method may apply the expansion force by application of apressurized inert gas such as nitrogen within the substrate lumen. Acompleted substrate may be placed inside a molding tube which has aninner diameter that is larger than the cast cylinder. A distal end ordistal portion of the cast cylinder may be clamped or otherwise closedand a pressure source may be coupled to a proximal end of the castcylinder. The entire assembly may be positioned over a nozzle whichapplies heat to either the length of the cast cylinder or to a portionof cast cylinder. The increase in diameter of the cast cylinder may thusrealign the molecular orientation of the cast cylinder to increase itsradial strength. After the diameter has been increased, the castcylinder may be cooled.

Once the processing has been completed on the polymeric substrate, thesubstrate may be further formed or machined to create a variety ofdevice. One example includes stents created from the cast cylinder bycutting along a length of the cylinder to create a rolled stent fordelivery and deployment within the patient vasculature. Another exampleincludes machining a number of portions to create a lattice or scaffoldstructure which facilitates the compression and expansion of the stent.

In other variations, in forming the stent, the substrate may be firstformed at a first diameter, as described herein by immersing a mandrelinto at least a first polymeric solution such that at least a firstlayer of a biocompatible polymer substrate is formed upon the mandreland has a first diameter defined by the mandrel. In forming thesubstrate, parameters such as controlling a number of immersions of themandrel into the first polymeric solution, controlling a duration oftime of each immersion of the mandrel, and controlling a delay timebetween each immersion of the mandrel are controlled. With the substrateinitially formed, the first diameter of the substrate may be reduced toa second smaller diameter and processed to form an expandable stentscaffold configured for delivery and deployment within a vessel, whereinthe stent scaffold retains one or more mechanical properties of thepolymer resin such that the stent scaffold exhibits ductility uponapplication of a load.

With the stent scaffold formed and heat set to have an initial diameter,it may be reduced to a second delivery diameter and placed upon adelivery catheter for intravascular delivery within a patient bodycomprising positioning the stent having the second diameter at a targetlocation within the vessel, expanding the stent to a third diameter thatis larger than the second diameter (and possibly smaller than theinitial diameter) at the target location utilizing an inflation balloonor other mechanism, and allowing the stent to then self-expand intofurther contact with the vessel at the target location such that thestent self-expands over time back to its initial diameter or until it isconstrained from further expansion by the vessel walls.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a stress-strain plot of polylactic acid (PLLA) atdiffering molecular weights and their corresponding stress-strain valuesindicating brittle fracture to ductile failure.

FIG. 2A illustrates an example of a dip-coating machine which may beutilized to form a polymeric substrate having one or more layers formedalong a mandrel.

FIGS. 2B and 2C illustrate another example of a dip-coating assemblyhaving one or more articulatable linkages to adjust a dipping directionof the mandrel.

FIGS. 3A to 3C show respective partial cross-sectional side and endviews of an example of a portion of a multi-layer polymeric substrateformed along the mandrel and the resulting substrate.

FIG. 4A illustrates an example of a resulting stress-strain plot ofvarious samples of polymeric substrates formed by a dip-coating processand the resulting plots indicating ductile failure.

FIG. 48 illustrates another example of a stress-strain plot ofadditional samples formed by dip-coating along with samplesincorporating a layer of BaSO₄.

FIG. 4C illustrates an example of a detailed end view of a PLLA 8.28substrate having a BaSO₄ layer incorporated into the substrate.

FIGS. 5A and 5B illustrate perspective views of an example of a dip-coatformed polymeric substrate undergoing plastic deformation and theresulting high percentage elongation.

FIG. 6 illustrates an example of an additional forming procedure where aformed polymeric substrate may be expanded within a molding or formingtube to impart a circumferential orientation into the substrate.

FIG. 7 illustrates another example of an additional forming procedurewhere a formed polymeric substrate may be rotated to induce acircumferentially-oriented stress value to increase the radial strengthof the substrate.

FIG. 8 illustrates a perspective view of one example of a rolled sheetstent which may be formed with the formed polymeric substrate.

FIG. 9 illustrates a side view of another example of a stent machinedvia any number of processes from the resulting polymeric substrate.

FIGS. 10A to 10F illustrate side views of another example of how a stentformed from a polymeric substrate may be delivered and deployedinitially via balloon expansion within a vessel and then allowed toself-expand further in diameter to its initial heat set diameter.

DETAILED DESCRIPTION OF THE INVENTION

In manufacturing implantable devices from polymeric materials such asbiocompatible and/or biodegradable polymers, a number of castingprocesses described herein may be utilized to develop substrates, e.g.,cylindrically shaped substrates, having a relatively high level ofgeometric precision and mechanical strength. These polymeric substratescan then be machined using any number of processes e.g., high-speedlaser sources, mechanical machining, etc.) to create devices such asstents having a variety of geometries for implantation within a patient,such as the peripheral or coronary vasculature, etc.

An example of such a casting process is to utilize a dip-coatingprocess. The utilization of dip-coating to create a polymeric substratehaving such desirable characteristics results in substrates which areable to retain the inherent properties of the starting materials. Thisin turn results in substrates having a relatively high radial strengthwhich is mostly retained through any additional manufacturing processesfor implantation. Additionally, dip-coating the polymeric substrate alsoallows for the creation of substrates having multiple layers. Themultiple layers may be formed from the same or similar materials or theymay be varied to include any number of additional agents, such as one ormore drugs for treatment of the vessel, as described in further detailbelow. Moreover, the variability of utilizing multiple layers for thesubstrate may allow one to control other parameters, conditions, orranges between individual layers such as varying the degradation ratebetween layers while maintaining the intrinsic molecular weight andmechanical strength of the polymer at a high level with minimaldegradation of the starting materials.

Because of the retention of molecular weight and mechanical strength ofthe starting materials via the casting or dip-coating process, polymericsubstrates may be formed which enable the fabrication of devices such asstents with reduced wall thickness which is highly desirable for thetreatment of arterial diseases. Furthermore these processes may producestructures having precise geometric tolerances with respect to wallthicknesses, concentricity, diameter, etc.

One mechanical property in particular which is generally problematicwith, e.g., polymeric stents formed from polymeric substrates, isfailure via brittle fracture of the device when placed under stresswithin the patient body. It is generally desirable for polymeric stentsto exhibit ductile failure under an applied load rather via brittlefailure, especially during delivery and deployment of a polymeric stentfrom an inflation balloon or constraining sheath, as mentioned above.Percent (%) ductility is generally a measure of the degree of plasticdeformation that has been sustained by the material at fracture. Amaterial that experiences very little or no plastic deformation uponfracture is brittle.

The molecular weight of a polymer is typically one of the factors indetermining the mechanical behavior of the polymer. With an increase inthe molecular weight of a polymer, there is generally a transition frombrittle to ductile failure. An example is illustrated in thestress-strain plot 10 which illustrate the differing mechanical behaviorresulting from an increase in molecular weight. The stress-strain curve12 of a sample of polylactic acid (PLLA) 2.4 shows a failure point 18having a relatively low tensile strain percentage at a high tensilestress level indicating brittle failure. A sample of PLLA 4.3, which hasa relatively higher molecular weight than PLLA 2.4, illustrates astress-strain curve 14 which has a region of plastic failure 20 afterthe onset of yielding and a failure point 22 which has a relativelylower tensile stress value at a relatively higher tensile strainpercentage indicating a degree of ductility. Yield occurs when amaterial initially departs from the linearity of a stress-strain curveand experiences an elastic-plastic transition.

A sample of PLLA 8.4, which has yet a higher molecular weight than PLLA4.3, illustrates a stress-strain curve 16 which has a longer region ofplastic failure 24 after the onset of yielding. The failure point 26also has a relatively lower tensile stress value at a relatively highertensile strain percentage indicating a degree of ductility. Thus, ahigh-strength tubular material which exhibits a relatively high degreeof ductility may be fabricated utilizing polymers having a relativelyhigh molecular weight (e.g., PLLA 8.4, PLLA with 8.28 IV, etc.). Such atubular material may be processed via any number of machining processesto form an implantable device such as a stent which exhibits astress-strain curve which is associated with the casting or dip-coatingprocess described herein.

An example of a mandrel which may be utilized to cast or dip-coat thepolymeric substrate is illustrated in the side view of FIG. 2A.Generally, dip coating assembly 30 may be any structure which supportsthe manufacture of the polymeric substrate in accordance with thedescription herein. A base 32 may support a column 34 which houses adrive column 36 and a bracket arm 38. Motor 42 may urge drive column 36vertically along column 34 to move bracket arm 38 accordingly. Mandrel40 may be attached to bracket arm 38 above container 44 which may befilled with a polymeric solution 46 (e.g., PLLA, PLA, PLGA, etc.) intowhich mandrel 40 may be dipped via a linear motion 52. The one or morepolymers may be dissolved in a compatible solvent in one or morecorresponding containers 44 such that the appropriate solution ma beplaced under mandrel 40. An optional motor 48 may be mounted alongbracket arm 38 or elsewhere along assembly 30 to impart an optionalrotational motion 54 to mandrel 40 and the substrate 50 formed alongmandrel 40 to impart an increase in the circumferential strength ofsubstrate 50 during the dip-coating process, as described in furtherdetail below.

The assembly 30 may be isolated on a vibration-damping or vibrationallyisolated table to ensure that the liquid surface held within container44 remains completely undisturbed to facilitate the formation of auniform thickness of polymer material along mandrel 40 and/or substrate50 with each deposition The entire assembly 30 or just a portion of theassembly such as the mandrel 40 and polymer solution may be placed in aninert environment such as a nitrogen gas environment while maintaining avery low relative humidity (RH) level, e.g., less than 30% RH, andappropriate dipping temperature, e.g., at least 20° C. below the boilingpoint of the solvent within container 44 so as to ensure adequatebonding between layers of the dip-coated substrate. Multiple mandrelsmay also be mounted along bracket arm 38 or directly to column 34.

The mandrel 40 may be sized appropriately and define a cross-sectionalgeometry to impart a desired shape and size to the substrate 50. Mandrel40 may be generally circular in cross section although geometries may beutilized as desired. In one example, mandrel 40 may define a circulargeometry having a diameter ranging from 1 mm to 20 mm to form apolymeric substrate having a corresponding inner diameter. Moreover,mandrel 40 may be made generally from various materials which aresuitable to withstand dip-coating processes, e.g., stainless steel,copper, aluminum, silver, brass, nickel, titanium, etc. The length ofmandrel 40 that is dipped into the polymer solution may be optionallylimited in length by, e.g., 50 cm, to ensure that an even coat ofpolymer is formed along the dipped length of mandrel 40 to limit theeffects of gravity during the coating process. Mandrel 40 may also bemade from a polymeric material which is lubricious, strong, has gooddimensional stability, and is chemically resistant to the polymersolution utilized for dip-coating, e.g., fluoropolymers, polyacetal,polyester, polyamide, polyacrylates, etc.

Moreover, mandrel 40 may be made to have a smooth surface for thepolymeric solution to form upon. In other variations, mandrel 40 maydefine a surface that is coated with a material such aspolytetrafluoromethylene to enhance removal of the polymeric substrateformed thereon. In yet other variations, mandrel 40 may be configured todefine any number of patterns over its surface, e.g., either over itsentire length or just a portion of its surface, that can bemold-transferred during the dip-coating process to the inner surface ofthe first layer of coating of the dip-coated substrate tube. Thepatterns may form raised or depressed sections to form various patternssuch as checkered, cross-hatched, cratered, etc. that may enhanceendothelialization with the surrounding tissue after the device isimplanted within a patient, e.g., within three months or ofimplantation.

The direction that mandrel 40 is dipped within polymeric solution 46 mayalso be alternated or changed between layers of substrate 50. In formingsubstrates having a length ranging from, e.g., 1 cm to 40 cm or longer,substrate 50 may be removed from mandrel 40 and replaced onto mandrel 40in an opposite direction before the dipping process is continued.Alternatively, mandrel 40 may be angled relative to bracket arm 38and/or polymeric solution 46 during or prior to the dipping process.

This may also be accomplished in yet another variation by utilizing adipping assembly as illustrated in FIGS. 2B and 2C to achieve a uniformwall thickness throughout the length of the formed substrate 50 per dip.For instance, after 1 to 3 coats are formed in a first dippingdirection, additional layers formed upon the initial layers may beformed by dipping mandrel 40 in a second direction opposite to the firstdipping direction, e.g., angling the mandrel 40 anywhere up to 180° fromthe first dipping direction. This may be accomplished in one examplethrough the use of one or more pivoting linkages 56, 58 connectingmandrel 40 to bracket arm 38, as illustrated. The one or more linkages56, 58 may maintain mandrel 40 in a first vertical position relative tosolution 46 to coat the initial layers of substrate 50, as shown in FIG.2B. Linkages 56, 58 may then be actuated to reconfigure mandrel 40 fromits first vertical position to a second vertical position opposite tothe first vertical position, as indicated by direction 59 in FIG. 2C.With repositioning of mandrel 40 complete, the dipping process may beresumed by dipping the entire linkage assembly along with mandrel 40 andsubstrate 50. In this manner, neither mandrel 40 nor substrate 50 needsto be removed and thus eliminates any risk of contamination. Linkages56, 58 may comprise any number of mechanical or electromechanicalpivoting and/or rotating mechanisms as known in the art.

Dipping mandrel 40 and substrate 50 in different directions may alsoenable the coated layers to have a uniform thickness throughout from itsproximal end to its distal end to help compensate for the effects ofgravity during the coating process. These values are intended to beillustrative and are not intended to be limiting in any manner. Anyexcess dip-coated layers on the linkages 56, 58 may simply be removedfrom mandrel 40 by breaking the layers. Alternating the dippingdirection may also result in the polymers being oriented alternatelywhich may reinforce the tensile strength in the axial direction of thedip coated tubular substrate 50.

With dip-coating assembly 30, one or more high molecular weightbiocompatible and/or bioabsorbable polymers may be selected for formingupon mandrel 40. Examples of polymers which may be utilized to form thepolymeric substrate may include, but is not limited to, polyethylene,polycarbonates, polyamides, polyesteramides, polyetheretherketone,polyacetals, polyketals, polyurethane, polyolefin, or polyethyleneterephthalate and degradable polymers, for example, polylactide (PLA)including poly-L-lactide (PLLA), poly-glycolide (PGA),poly(lactide-co-glycolide) (PLGA) or polycaprolactone, caprolactones,polydioxanones, polyanhydrides, polyorthocarbonates, polyphosphazenes,chitin, chitosan, poly(amino acids), and polyorthoesters, andcopolymers, terpolymers and combinations and mixtures thereof.

Other examples of suitable polymers may include synthetic polymers, forexample, oligomers, homopolymers and co-polymers, acrylics such as thosepolymerized from methyl cerylate, methyl methacrylate, acryli acid,methacrylic acid, acrylamide, hydroxyethy acrylate, hydroxyethylmethacrylate, glyceryl serylate, glyceryl methacrylate, methacrylamideand ethacrylamide; vinyls such as styrene, vinyl chloride, binalypyrrolidone, polyvinyl alcohol, and vinyls acetate; polymers formed ofethylene, propylene, and tetrfluoroethylene. Further examples mayinclude nylons such as polycoprolactam, polylauryl lactam,polyjexamethylene adipamide, and polyexamethylene dodecanediamide, andalso polyurethanes, polycarbonates, polyamides, polysulfones,poly(ethylene terephthalate), polyactic acid, polyglycolic acid,polydimethylsiloxanes, and polyetherketones.

Examples of biodegradable polymers which can be used for dip-coatingprocess are polylactide (PLA), polyglycolide (PGA),poly(lactide-co-glycolide) (PLGA), poly(e-caprolactone), polydioxanone,polyanhydride, trimethylene carbonate, poly(β-hydroxybutyrate),poly(g-ethyl glutamate), poly(DTH iminocarbonate), poly(bisphenol Aiminocarbonate), poly(ortho ester), polycyanoacrylate, andpolyphosphazene, and copolymers, terpolymers and combinations andmixtures thereof. There are also a number of biodegradable polymersderived from natural sources such as modified polysaccharides(cellulose, chitin, chitosan, dextran) or modified proteins (fibrin,casein).

Other examples of suitable polymers may include synthetic polymers, forexample, oligomers, homopolymers, and co-polymers, acrylics such asthose polymerized from methyl cerylate, methyl methacrylate, acryliacid, methacrylic acid, acrylamide, hydroxyethy acrylate, hydroxyethylmethacrylate, glyceryl scrylate, glyceryl methacrylate, methacrylamideand ethacrylamide, vinyls such as styrene, vinyl chloride, binalypyrrolidone, polyvinyl alcohol, and vinyls acetate; polymers formed ofethylene, propylene, and tetrfluoroethylene. Further examples mayinclude nylons such as polycoprolactam, polylauryl lactam,polyjexamethylene adipamide, and polyexamethylene dodecanediamide, andalso polyurethanes, polycarbonates, polyamides, polysulfones,poly(ethylene terephthalate), polyacetals, polyketals,polydimethylsiloxanes, and polyetherketones.

These examples of polymers which may be utilized for forming thesubstrate are not intended to be limiting or exhaustive but are intendedto be illustrative of potential polymers which may be used. As thesubstrate may be formed to have one or more layers overlaid upon oneanother, the substrate may be formed to have a first layer of a firstpolymer, a second layer of a second polymer, and so on depending uponthe desired structure and properties of the substrate. Thus, the varioussolutions and containers may be replaced beneath mandrel 40 betweendip-coating operations in accordance with the desired layers to beformed upon the substrate such that the mandrel 40 may be dippedsequentially into the appropriate polymeric solution.

Depending upon the desired wall thickness of the formed substrate, themandrel 40 may be dipped into the appropriate solution as determined bythe number of times the mandrel 40 is immersed, the duration of time ofeach immersion within the solution, as well as the delay time betweeneach immersion or the drying or curing time between dips. Additionally,parameters such as the dipping and/or withdrawal rate of the mandrel 40from the polymeric solution may also be controlled to range from, e.g.,5 mm/min to 1000 mm/min. Formation via the dip-coating process mayresult in a polymeric substrate having half the wall thickness whileretaining an increased level of strength in the substrate as compared toan extruded polymeric structure. For example, to form a substrate havinga wall thickness of, e.g., 200 μm, built up of multiple layers ofpolylactic acid, mandrel 40 may be dipped between, e.g., 2 to 20 timesor more, into the polymeric solution with an immersion time rangingfrom, e.g., 15 seconds (or less) to 240 minutes or more. Moreover, thesubstrate and mandrel 40 may be optionally dried or cured for a periodof time ranging from, e.g., 15 seconds (or less) to 60 minutes (or more)between each immersion. These values are intended to be illustrative andare not intended to be limiting in any manner.

Aside from utilizing materials which are relatively high in molecularweight, another parameter which may be considered m further increasingthe ductility of the material is its crystallinity, which refers to thedegree of structural order in the polymer. Such polymers may contain amixture of crystalline and amorphous regions where reducing thepercentage of the crystalline regions in the polymer may furtherincrease the ductility of the material. Polymeric materials not onlyhaving a relatively high molecular weight but also having a relativelylow crystalline percentage may be utilized in the processes describedherein to form a desirable tubular substrate.

The following Table 1 show examples of various polymeric materials(e.g., PLLA IV 8.28 and PDLLA 96/4) to illustrate the molecular weightsof the materials in comparison to their respective crystallinitypercentage. The glass transition temperature, T_(g), as well as meltingtemperature, T_(m), are given as well. An example of PLLA IV 8.28 isshown illustrating the raw resin and tube form as having the samemolecular weight, M_(w), of 1.70×10⁶ gram/mol. However, thecrystallinity percentage of PLLA IV 8.28 Resin is 61.90% while thecorresponding Tube form is 38.40%. Similarly for PDLLA 96/4, the resinform and tube form each have a molecular weight, M_(w), of 9.80×10⁵gram/mol; however, the crystallinity percentages are 46.20% and 20.90%,respectively.

TABLE 1 Various polymeric materials and their respective crystallinitypercentages. Crystallinity M_(w) Material T_(g) (° C.) T_(m) (° C.) (%)(gram/mol) PLLA IV8.28 Resin 72.5 186.4 61.90% 1.70 × 10⁶ PLLA IV8.28Tubes 73.3 176.3 38.40% 1.70 × 10⁶ PDLLA 96/4 Resin 61.8 155.9 46.20%9.80 × 10⁵ PDLLA 96/4 Tubes 60.3 146.9 20.90% 9.80 × 10⁵

As the resin is dip coated to form the tubular substrate through themethods described herein, the drying procedures and processing helps topreserve the relatively high molecular weight of the polymer from thestarting material and throughout processing to substrate and stentformation. Moreover, the drying processes in particular may facilitatethe formation of desirable crystallinity percentages, as describedabove.

Aside from the crystallinity of the materials, the immersion times aswell as drying times may be uniform between each immersion or they maybe varied as determined by the desired properties of the resultingsubstrate. Moreover, the substrate may be placed in an oven or dried atambient temperature between each immersion or after the final immersionto attain a predetermined level of crystals, e.g., 60%, and a level ofamorphous polymeric structure, e.g., 40%. Each of the layers overlaidupon one another during the dip-coating process are tightly adhered toone another and the mechanical properties of each polymer are retainedin their respective layer with no limitation on the molecular weight ofthe polymers utilized.

Varying the drying conditions of the materials may also be controlled toeffect desirable material parameters. The polymers may be dried at orabove the glass transition temperature (e.g., 10° to 20° C. above theglass transition temperature, T_(g)) of the respective polymer toeffectively remove any residual solvents from the polymers to attainresidual levels of less than 100 ppm, e.g., between 20 to 100 ppm.Positioning of the polymer substrate when drying is another Factor whichmay be controlled as affecting parameters, such as geometry, of thetube. For instance, the polymer substrate may be maintained in a dryingposition such that the substrate tube is held in a perpendicularposition relative to the ground such that the concentricity of the tubesis maintained. The substrate tube may be dried in an oven at or abovethe glass transition temperature, as mentioned, for a period of timeranging anywhere from, e.g., 10 days to 30 days or more. However,prolonged drying for a period of time, e.g., greater than 40 days, mayresult in thermal degradation of the polymer material.

Additionally and optionally, a shape memory effect may be induced in thepolymer during drying of the substrate. For instance, a shape memoryeffect may be induced in the polymeric tubing to set the tubular shapeat the diameter that was formed during the dip-coating process. Anexample of this is to form a polymeric tube by a dip-coating processdescribed herein at an outer diameter of 5 mm and subjecting thesubstrate to temperatures above its glass transition temperature, T_(g).At its elevated temperature, the substrate may be elongated, e.g., froma length of 5 cm to 7 cm, while its outer diameter of 5 mm is reduced to3 mm. Of course, these examples are merely illustrative and the initialdiameter may generally range anywhere from, e.g., 3 mm to 9 mm, and thereduced diameter may generally range anywhere from, e.g., 1.5 mm to 5mm, provided the reduced diameter is less than the initial diameter.

Once lengthened and reduced in diameter, the substrate may be quenchedor cooled in temperature to a sub-Tg level, e.g., about 20° C. below itsTg, to allow for the polymeric substrate to transition back to its glassstate. This effectively imparts a shape memory effect of self-expansionto the original diameter of the substrate. When such a tube (or stentformed from the tubular substrate) is compressed or expanded to asmaller or larger diameter and later exposed to an elevated temperature,over time the tube (or stent) may revert to its original 5 mm diameter.This post processing may also be useful for enabling self-expansion ofthe substrate after a process like laser cutting (e.g., when formingstents or other devices for implantation within the patient) where thesubstrate tube is typically heated to its glass transition temperature,Tg.

An example of a substrate having multiple layers is illustrated in FIGS.3A and 3B which show partial cross-sectional side views of an example ofa portion of a multi-layer polymeric substrate formed along mandrel 40and the resulting substrate. Substrate 50 may be formed along mandrel 40to have a first layer 60 formed of a first polymer, e.g.,poly(l-lactide). After the formation of first layer 60, an optionalsecond layer 62 of polymer, e.g., poly(L-lactide-co-glycolide), may beformed upon first layer 60. Yet another optional third layer 64 ofpolymer, e.g., poly(d,l-lactide-co-glycolide), may be formed upon secondlayer 62 to form a resulting substrate defining a lumen 66 therethroughwhich may be further processed to form any number of devices, such as astent. One or more of the layers may be formed to degrade at a specifiedrate or to elute any number of drugs or agents.

An example of this is illustrated in the cross-sectional end view ofFIG. 3C, which shows an exemplary substrate having three layers 60, 62,64 formed upon one another, as above. In this example, first layer 60may have a molecular weight of M_(n1), second layer 62 may have amolecular weight of M_(n2), and third layer 64 may have a molecularweight of M_(n3). A stent fabricated from the tube may be formed suchthat the relative molecular weights are such where M_(n1)>M_(n2)>M_(n3)to achieve a preferential layer-by-layer degradation through thethickness of the tube beginning with the inner first layer 60 andeventually degrading to the middle second layer 62 and finally to theouter third layer 64 when deployed within the patient body.Alternatively, the stent may be fabricated where the relative molecularweights are such where M_(n1)<M_(n2)<M_(n3) to achieve a layer-by-layerdegradation beginning with the outer third layer 64 and degradingtowards the inner first layer 60. This example is intended to beillustrative and fewer than or more than three layers may be utilized inother examples. Additionally, the molecular weights of each respectivelayer may be altered in other examples to vary the degradation ratesalong different layers, if so desired.

Moreover, any one or more of the layers may be formed to impartspecified mechanical properties to the substrate 50 such that thecomposite mechanical properties of the resulting substrate 50 mayspecifically tuned or designed. Additionally, although three layers areillustrated in this example, any number of layers may be utilizeddepending upon the desired mechanical properties of the substrate 50.

Moreover, as multiple layers may be overlaid one another in forming thepolymeric substrate, specified layers may be designated for a particularfunction in the substrate. For example, in substrates which are used tomanufacture polymeric stents, one or more layers may be designed asload-bearing layers to provide structural integrity to the stent whilecertain other layers may be allocated for drug-loading or eluting. Thoselayers which are designated for structural support may be formed fromhigh-molecular weight polymers, e.g., PLLA or any other suitable polymerdescribed herein, to provide a high degree of strength by omitting anydrugs as certain pharmaceutical agents may adversely affect themechanical properties of polymers. Those layers which are designated fordrug-loading may be placed within, upon, or between the structurallayers.

Additionally, multiple layers of different drugs may be loaded withinthe various layers. The manner and rate of drug release from multiplelayers may depend in part upon the degradation rates of the substratematerials. For instance, polymers which degrade relatively quickly mayrelease their drugs layer-by-layer as each successive layer degrades toexpose the next underlying layer. In other variations, drug release maytypically occur from a multilayer matrix via a combination of diffusionand degradation. In one example, a first layer may elute a first drugfor, e.g., the first 30 to 40 days after implantation. Once the firstlayer has been exhausted or degraded, a second underlying layer having asecond drug may release this drug for the next 30 to 40 days, and so onif so desired. In the example of FIG. 3B, for a stent (or otherimplantable device) manufactured from substrate 50, layer 64 may containthe first drug for release while layer 62 may contain the second drugfor release after exhaustion or degradation of layer 64. The underlyinglayer 60 may omit any pharmaceutical agents to provide uncompromisedstructural support to the entire structure.

In other examples, rather than having each successive layer elute itsrespective drug, each layer 62, 64 (optionally layer 60 as well), mayelute its respective drug simultaneously or at differing rates via acombination of diffusion and degradation. Although three layers areillustrated in this example, any number of layers may be utilized withany practicable combination of drugs for delivery. Moreover, the releasekinetics of each drug from each layer may be altered in a variety ofways by changing the formulation of the drug-containing layer.

Examples of drugs or agents which may be loaded within certain layers ofsubstrate 50 may include one or more antipoliferative, antineoplastic,antigenic, anti-inflammatory, and/or antirestenotic agents. Thetherapeutic agents may also include antilipid, antimitotics,metalloproteinase inhabitors, anti-sclerosing agents. Therapeutic agentsmay also include peptides, enzymes, radio isotopes or agents for avariety of treatment options. This list of drugs or agents is presentedto be illustrative and is not intended to be limiting.

Similarly certain other layers may be loaded with radio-opaquesubstances such as platinum, gold, etc. to enable visibility of thestent under imaging modalities such as fluoroscopic imaging.Radio-opaque substances like tungsten, platinum, gold, etc. can be mixedwith the polymeric solution and dip-coated upon the substrate such thatthe radio-opaque substances form a thin sub-micron thick layer upon thesubstrate. The radio-opaque substances may thus become embedded withinlayers that degrade in the final stages of degradation or within thestructural layers to facilitate stent visibility under an imagingmodality, such as fluoroscopy, throughout the life of the implanteddevice before fully degrading or losing its mechanical strength.Radio-opaque marker layers can also be dip-coated at one or both ends ofsubstrate 50, e.g., up to 0.5 mm from each respective end. Additionally,the radio-opaque substances can also be spray-coated or cast along aportion of the substrate 50 between its proximal and distal ends in aradial direction by rotating mandrel 40 when any form of radio-opaquesubstance is to be formed along any section of length of substrate 50.Rings of polymers having radio-opaque markers can also be formed as partof the structure of the substrate 50.

In an experimental example of the ductility and retention of mechanicalproperties, PLLA with Iv 8.4 (high molecular weight) was obtained andtubular substrates were manufactured utilizing the dip-coating processdescribed herein. The samples were formed to have a diameter of 5 mmwith a wall thickness of 200 μm and were comprised of 6 layers of PLLA8.4. The mandrel was immersed 6 times into the polymeric solution andthe substrates were dried or cured in an oven to obtain a 60%crystalline structure. At least two samples of tubular substrates weresubjected to tensile testing and stress-strain plot 70 was generatedfrom the stress-strain testing, as shown in FIG. 4A.

As shown in plot 70, a first sample of PLLA 8.4 generated astress-strain curve 72 having a region of plastic failure 76 where thestrain percentage increased at a relatively constant stress value priorto failure indicating a good degree of sample ductility. A second sampleof PLLA 8.4 also generated a stress-strain curve 74 having a relativelygreater region of plastic failure 78 also indicating a good degree ofsample ductility.

Polymeric stents and other implantable devices made from such substratesmay accordingly retain the material properties from the dip-coatedpolymer materials. The resulting stents, for instance, may exhibitmechanical properties which have a relatively high percentage ductilityin radial, torsional, and/or axial directions. An example of this is aresulting stent having an ability to undergo a diameter reduction ofanywhere between 5% to 70% when placed under an external load withoutany resulting plastic deformation. Such a stent may also exhibit highradial strength with, e.g., a 20% radial deformation when placed under a0.1 N to 20 N load. Such a stent may also be configured to self-expandwhen exposed to normal body temperatures.

The stent may also exhibit other characteristic mechanical propertieswhich are consistent with a substrate formed as described herein, forinstance, high ductility and high strength polymeric substrates. Suchsubstrates (and processed stents) may exhibit additional characteristicssuch as a percent reduction in diameter of between 5% to 70% withoutfracture formation when placed under a compressive load as well as apercent reduction in axial length of between 10% to 30% without fractureformation when placed under an axial load. Because of the relativelyhigh ductility, the substrate or stent may also be adapted to curve upto 180° about a 1 cm curvature radius without fracture formation orfailure. Additionally, when deployed within a vessel, a stent may alsobe expanded, e.g., by an inflatable intravascular balloon, by up to 5%to 70% to regain diameter without fracture formation or failure.

These values are intended to illustrate examples of how a polymerictubing substrate and a resulting stent may be configured to yield adevice with certain mechanical properties. Moreover, depending upon thedesired results, certain tubes and stents may be tailored for specificrequirements of various anatomical locations within a patient body byaltering the polymer and/or copolymer blends to adjust variousproperties such as strength, ductility, degradation rates, etc.

FIG. 4B illustrates a plot 71 of additional results from stress-straintesting with additional polymers. A sample of PLLA 8.28 was formedutilizing the methods described herein and tested to generatestress-strain curve 73 having a point of failure 73′. Additional samplesof PLLA 8.28 each with an additional layer of BaSO₄ incorporated intothe tubular substrate were also formed and tested. A first sample ofPLLA 8.28 with a layer of BaSO₄ generated stress-strain curve 77 havinga point of failure 77′. A second sample of PLLA 8.28 also with a layerof BaSO₄ generated stress-strain curve 79 having a point of failure 79′,which showed a greater tensile strain than the first sample with aslightly higher tensile stress level. A third sample of PLLA 8.28 with alayer of BaSO₄ generated stress-strain curve 81 having a point offailure 81′, which was again greater than the tensile strain of thesecond sample, yet not significantly greater than the tensile stresslevel. The inclusion of BaSO₄ may accordingly improve the elasticmodulus values of the polymeric substrates. The samples of PLLA 8.28generally resulted in a load of between 100 N to 300 N at failure of thematerials, which yielded elastic modulus values of between 1000 to 3000MPa with a percent elongation of between 10% to 300% at failure.

A sample of 96/4 PDLLA was also formed and tested to generatestress-strain curve 75 having a point of failure 75′ which exhibited arelatively lower percent elongation characteristic of brittle fracture.The resulting load at failure was between 100 N to 300 N with an elasticmodulus of between 1000 to 3000 MPa, which was similar to the PLLA 8.28samples. However, the percent elongation was between 10% to 40% atfailure.

FIG. 4C illustrates an example of a detailed end view of a PLLA 8.28substrate 83 formed with multiple dip-coated layers via a processdescribed herein as viewed under a scanning electron microscope. Thisvariation has a BaSO₄ layer 85 incorporated into the substrate. Asdescribed above, one or more layers of BaSO₄ may be optionallyincorporated into substrate 83 to alter the elastic modulus of theformed substrate. Additionally, the individual layers overlaid atop oneanother are fused to form a single cohesive layer rather than multipleseparate layers as a result of the drying processes during the dippingprocess described herein. This results in a unitary structure whichfurther prevents or inhibits any delamination from occurring between theindividual layers.

FIGS. 5A and 5B illustrate perspective views of one of the samples whichwas subjected to stress-strain testing on tensile testing system 80. Thepolymeric substrate specimen 86 was formed upon a mandrel, as describedabove, into a tubular configuration and secured to testing platform 82,84. With testing platform 82, 84 applying tensile loading, substratespecimen 86 was pulled until failure. The relatively high percentage ofelongation is illustrated by the stretched region of elongation 88indicating a relatively high degree of plastic deformation when comparedto an extruded polymeric substrate. Because a polymeric substrate formedvia dip-coating as described above may be reduced in diameter viaplastic deformation without failure, several different stent diameterscan be manufactured from a single diameter substrate tube.

Dip-coating can be used to impart an orientation between layers (e.g.,linear orientation by dipping; radial orientation by spinning themandrel; etc.) to further enhance the mechanical properties of theformed substrate. As radial strength is a desirable attribute of stentdesign, post-processing of the formed substrate may be accomplished toimpart such attributes. Typically, polymeric stents suffer from havingrelatively thick walls to compensate for the lack of radial strength,and this in turn reduces flexibility, impedes navigation, and reducesarterial luminal area immediately post implantation. Post-processing mayalso help to prevent material creep and recoil (creep is atime-dependent permanent deformation that occurs to a specimen understress, typically under elevated temperatures) which are problemstypically associated with polymeric stents.

In further increasing the radial or circumferential strength of thepolymeric substrate, a number of additional processes may be applied tothe substrate after the dip-coating procedure is completed (or close tobeing completed). A polymer that is amorphous or that is partiallyamorphous will generally undergo a transition from a pliable, elasticstate (at higher temperatures) to a brittle glass-like state at lowertemperature as it transitions through a particular temperature, referredas the glass transition temperature (T_(g)). The glass transitiontemperature for a given polymer will vary, depending on the size andflexibility of side chains, as well as the flexibility of the backbonelinkages and the size of functional groups incorporated into the polymerbackbone. Below T_(g), the polymer will maintain some flexibility, andmay be deformed to a new shape. However, the further the temperaturebelow T_(g) the polymer is when being deformed, the greater the forceneeded to shape it.

Moreover, when a polymer is in glass transition temperature itsmolecular structure can be manipulated to form an orientation in adesired direction. Induced alignment of polymeric chains or orientationimproves mechanical properties and behavior of the material. Molecularorientation is typically imparted by application of force while thepolymer is in a pliable, elastic state. After sufficient orientation isinduced, temperature of the polymer is reduced to prevent reversal anddissipation of the orientation.

In one example, the polymeric substrate may be heated to increase itstemperature along its entire length or along a selected portion of thesubstrate to a temperature that is at or above the T_(g) of the polymer.For instance, for a substrate fabricated from PLLA, the substrate may beheated to a temperature between 60° C. to 70° C. Once the substrate hasreached a sufficient temperature such that enough of its molecules havebeen mobilized, a force may be applied from within the substrate oralong a portion of the substrate to increase its diameter from a firstdiameter D₁ to a second increased diameter D₂ for a period of timenecessary to set the increased diameter. During this setting period, theapplication of force induces a molecular orientation in acircumferential direction to align the molecular orientation of polymerchains to enhance its mechanical properties. The re-formed substrate maythen be cooled to a lower temperature typically below T_(g), forexample, by passing the tube through a cold environment, typically dryair or an inert gas to maintain the shape at diameter D₂ and preventdissipation of molecular orientation.

The force applied to the substrate may be generated by a number ofdifferent methods. One method is by utilizing an expandable pressurevessel placed within the substrate. Another method is by utilizing abraid structure, such as a braid made from a super-elastic or shapememory alloy like NiTi alloy, to increase in size and to apply thedesirable degree of force against the interior surface of the substrate.

Yet another method may apply the expansion force by application of apressurized inert gas such as nitrogen within the substrate lumen, asshown in FIG. 6, to impart a circumferential orientation in thesubstrate. A completed substrate, e.g., cast cylinder 94, may be placedinside a molding tube 90 which has an inner diameter that is larger thanthe cast cylinder 94. Molding tube 90 may be fabricated from glass,highly-polished metal, or polymer. Moreover, molding tube 90 may befabricated with tight tolerances to allow for precision sizing of castcylinder 94.

A distal end or distal portion of cast cylinder 94 may be clamped 96 orotherwise closed and a pressure source may be coupled to a proximal end98 of cast cylinder 94. The entire assembly may be positioned over anozzle 102 which applies heat 104 to either the length of cast cylinder94 or to a portion of cast cylinder 94. The pressurized inert gas 100,e.g., pressured to 10 to 400 psi, may be introduced within cast cylinder94 to increase its diameter, e.g., 2 mm, to that of the inner diameter,e.g., 4 mm, of molding tube 90. The increase in diameter of castcylinder 94 may thus realign the molecular orientation of cast cylinder94 to increase its radial strength and to impart a circumferentialorientation in the cast cylinder 94. Portion 92 illustrates radialexpansion of the cast cylinder 94 against the inner surface of themolding tube 90 in an exaggerated manner to illustrate the radialexpansion and impartation of circumferential strength. After thediameter has been increased, cast cylinder 94 may be cooled, asdescribed above.

Once the substrate has been formed and reduced in diameter to itssmaller second diameter, the stent may be processed, as described above.Alternatively, the stent may be processed from the substrate afterinitial formation. The stent itself may then be reduced in diameter toits second reduced diameter.

In either case, once the stent has been formed into its second reduceddiameter, the stent may be delivered to a targeted location within avessel of a patient. Delivery may be effected intravascularly utilizingknown techniques with the stent in its second reduced delivery diameterpositioned upon, e.g., an inflation balloon, for intravascular delivery.Once the inflation catheter and stent has been positioned adjacent tothe targeted region of vessel, the stent may be initially expanded intocontact against the interior surface of the vessel.

With the stent expanded into contact against the vessel wall at a thirddiameter which is larger than the second delivery diameter, theinflation balloon may be removed from the stent. Over a predeterminedperiod of time and given the structural characteristics of the stent,the stent may then also self-expand further into contact against thevessel wall for secure placement and positioning.

Because thermoplastic polymers such as PLLA typically soften whenheated, the cast cylinder 94 or a portion of the cast cylinder 94 may beheated in an inert environment, e.g., a nitrogen gas environment, tominimize its degradation.

Another method for post-processing a cast cylinder 110 may be seen inthe example of FIG. 7 for inducing a circumferential orientation in theformed substrate. As illustrated, mandrel 112 having the cast cylinder110 may be re-oriented into a horizontal position immediately postdip-coating before the polymer is cured. Mandrel 112 may be rotated, asindicated by rotational movement 116, at a predetermined speed, e.g., 1to 300 rpm, while the cylinder 110 is heated via nozzle 102. Mandrel 112may also be optionally rotated via motor 48 of assembly 30 to impart therotational motion 54, as shown above in FIG. 2. Mandrel 112 may also bemoved in a linear direction 114 to heat the length or a portion of thelength of the cylinder 110. As above, this post-processing may becompleted in an inert environment.

Once the processing has been completed on the polymeric substrate, thesubstrate may be further formed or machined to create a variety ofdevice. One example is shown in the perspective view of FIG. 8, whichillustrates rolled stent 120. Stent 120 may be created from the castcylinder by cutting along a length of the cylinder to create anoverlapping portion 122. The stent 120 may then be rolled into a smallconfiguration for deployment and then expanded within the patientvasculature. Another example is illustrated in the side view of stent124, which may be formed by machining a number of removed portions 126to create a lattice or scaffold structure which facilitates thecompression and expansion of stent 124 for delivery and deployment.

FIGS. 10A to 10F illustrate side views of another example of how a stent130 formed from a polymeric substrate may be delivered and deployed forsecure expansion within a vessel. FIG. 10A shows a side view of anexemplary stent 130 which has been processed or cut from a polymericsubstrate formed with an initial diameter D1. As described above, thesubstrate may be heat treated at, near, or above the glass transitiontemperature T_(g) of the substrate to set this initial diameter D1 andthe substrate may then be processed to produce the stent 130 such thatthe stent 130 has a corresponding diameter D1. Stent 130 may then bereduced in diameter to a second delivery diameter D2 which is less thanthe initial diameter D1 such that the stent 130 may be positioned upon,e.g., an inflation balloon 134 of a delivery catheter 132, as shown inFIG. 10B. The stent 130 at its reduced diameter D2 may beself-constrained such that the stent 130 remains in its reduced diameterD2 without the need for an outer sheath, although a sheath may beoptionally utilized. Additionally, because of the processing and theresultant material characteristics of the stent material, as describedabove, the stent 130 may be reduced from initial diameter D1 to deliverydiameter D2 without cracking or material failure.

With stent 130 positioned upon delivery catheter 132, it may be advancedintravascularly within a vessel 136 until the delivery site is reached,as shown in FIG. 10C. Inflation balloon 134 may be inflated to expand adiameter of stent 130 into contact against the vessel interior, e.g., toan intermediate diameter D3, which is less than the stent's initialdiameter D1 yet larger than the delivery diameter D2. Stent 130 may beexpanded to this intermediate diameter D3 without any cracking orfailure because of the inherent material characteristics describedabove. Moreover, expansion to intermediate diameter D3 may allow for thestent 130 to securely contact the vessel wall while allowing for thewithdrawal of the delivery catheter 132, as shown in FIG. 10E.

Once the stent 130 has been expanded to some intermediate diameter D3and secured against the vessel wall, stent 130 may be allowed to thenself-expand further over a period of time into further contact with thevessel wall such that stent 130 conforms securely to the tissue. Thisself-expansion feature ultimately allows for the stent 130 to expandback to its initial diameter D1 which had been heat set, as shown inFIG. 10F, or until stent 130 has fully self-expanded within the confinesof the vessel diameter.

These examples are presented to be illustrative of the types of deviceswhich may be formed and various other devices which may be formed fromthe polymeric substrate are also included within this disclosure.

The applications of the disclosed invention discussed above are notlimited to certain processes, treatments, or placement in certainregions of the body, but may include any number of other processes,treatments, and areas of the body. Modification of the above-describedmethods and devices for carrying out the invention, and variations ofaspects of the invention that are obvious to those of skill in the artsare intended to be within the scope of this disclosure. Moreover,various combinations of aspects between examples are also contemplatedand are considered to be within the scope of this disclosure as well.

What is claimed is:
 1. A polymeric stent, comprising: a stent scaffoldformed from a polymeric substrate, wherein the polymeric substrate isformed from a polymeric solution having an inherent viscosity of about4.3 dl/g to about 8.4 dl/g, wherein the scaffold has a delivery diameterwhen secured upon a catheter and an expanded diameter when deployed fromthe catheter, where the expanded diameter is larger than the deliverydiameter, wherein the stent scaffold is formed by multiple dip-coatedlayers of the polymeric substrate, wherein a degree of crystallinity ofthe multiple dip-coated layers is between about 20% and 46%, at leastone layer of barium sulfate incorporated into the polymeric substrate,and wherein the scaffold is configured to self-expand by reverting overa predetermined period of time to its expanded diameter via a shapememory effect imparted to the scaffold when deployed after being reducedin diameter by up to 83%.
 2. The stent of claim 1 wherein the expandeddiameter of the scaffold is defined by a mandrel upon which thepolymeric substrate is formed.
 3. The stent of claim 1 wherein thepolymeric substrate has a length of 1 to 40 cm.
 4. The stent of claim 1wherein the expanded diameter of the scaffold is between 3 to 9 mm. 5.The stent of claim 1 wherein the scaffold is adapted to curve up to 180°about a 1 cm curvature radius without fracture formation.
 6. The stentof claim 1 wherein the stent is adapted to exhibit a percent reductionin axial length of between 10% to 30% without fracture formation whenplaced under an axial load.
 7. The stent of claim 1, wherein thescaffold is configured to self-expand at normal body temperatures byreverting over the predetermined period of time to its expanded diametervia a shape memory effect imparted to the scaffold when deployed.
 8. Thestent of claim 7, wherein said normal body temperatures is between about36° C. and 38° C.
 9. A polymeric stent, comprising: a stent scaffoldformed from a polymeric substrate, wherein the polymeric substrate isformed from a polymeric solution having an inherent viscosity of about4.3 dl/g to about 8.4 dl/g, wherein the scaffold has a delivery diameterwhen secured upon a catheter and an expanded diameter when deployed fromthe catheter, where the expanded diameter is larger than the deliverydiameter, wherein the stent scaffold is formed by multiple dip-coatedlayers of the polymeric substrate, wherein a degree of crystallinity ofthe multiple dip-coated layers is between about 20% and 46%, at leastone layer of barium sulfate incorporated into the polymeric substrate,and wherein the scaffold is configured to self-expand by reverting overa predetermined period of time to its expanded diameter via a shapememory effect imparted to the scaffold when deployed after being reducedin diameter between 5% to 70%.
 10. The stent of claim 9 wherein theexpanded diameter of the scaffold is defined by a mandrel upon which thepolymeric substrate is formed.
 11. The stent of claim 9 wherein thepolymeric substrate has a length of 1 to 40 cm.
 12. The stent of claim 9wherein the expanded diameter of the scaffold is between 3 to 9 mm. 13.The stent of claim 9 wherein the scaffold is adapted to curve up to 180°about a 1 cm curvature radius without fracture formation.
 14. The stentof claim 9 wherein the stent is adapted to exhibit a percent reductionin axial length of between 10% to 30% without fracture formation whenplaced under an axial load.
 15. The stent of claim 9, wherein thescaffold is configured to self-expand at normal body temperatures byreverting over the predetermined period of time to its expanded diametervia a shape memory effect imparted to the scaffold when deployed. 16.The stent of claim 15, wherein said normal body temperatures is betweenabout 36° C. and 38° C.